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Materials

The Geometry Center's icosahedron was made from 1/8 inch foamboard, which is available at most art supply stores. We used white foamboard and had the students color and decorate each panel. The amount of foamboard you need depends on the edge length for your icosahedron. Foamboard comes in rectangles of length 32x40 inches. Because foamboard costs approximately $5 per sheet, it is convenient to plan the side length of your icosahedron to maximize the number of triangles that will fit on each sheet. This makes a nice exercise for the students. For example:

The Geometry Center cut out each triangular panel from a 32x40 inch rectangular pieces of foamboard. Three possible ways to cut the triangle are shown in Figure 2. What is the maximum edge length of each triangle? Does your answer change if we also want to reduce the amount of cutting that we are required to do?
How many pieces of 32x40 inch foamboard do you need to buy in order to cut out 20 equilateral triangles if the edge length is 9 inches? 12 inches? 18 inches? A few ways to arrange equilateral triangles on a rectangle are given in Figure 3.

You will also need an X-acto knife ($2-$5), a one-hole paper punch, and "twist ties" (e.g., those used to close kitchen garbage bags). Hardware stores often have packs of 100 ties for about $1. You may also want materials for coloring; we used assorted magic markers. A yardstick, meterstick, or some other straightedge is also useful.

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Author: Frederick J. Wicklin <fjw@geom.umn.edu>
Comments to: webmaster@geom.umn.edu
Created: Summer 1994 --- Last modified: Jul 21 1996
Copyright © 1994-1996 by The Geometry Center All rights reserved.